Parallelism is achieving balance in sentences. When you present similar points in a sentence, those points should have similar structure. This means that if you have a list of adjectives, that each word you use in the list is an adjective. If you are presenting a series of verbs, the verbs all use the same form and tense. When sentences aren't parallel they are often very awkward and difficult to read. Readers should not be distracted from the meaning of your writing because your sentences aren't understandable.
Parallelism is achieved by finding the list within a sentence and then checking to see what type of structure best suits your meaning. Sometimes a preposition can introduce a list of three objects; at other times, each object will need its own preposition. You make those decisions when you evaluate the sentence you are trying to make parallel.